This invention relates to thermal magneto-optic storage mediums and in particular to provision of a biasing field for obtaining desired magnetizations in a storage layer.
In many magneto-optical storage mediums, data is represented by a magnetized domain. The magnetization direction is perpendicular to the medium which is usually a very thin magnetic film. The data is read based on what is called the magneto-optical effect. Basically, the magneto-optical effect is the shift of polarization direction of a polarized light beam when it encounters a magnetized medium as a function of the direction of magnetization of the domain. The polarized light beam is commonly provided by a laser.
To obtain magnetized domains representative of data, a laser is commonly used to heat the magnetic medium in the area where such a domain is desired. The magnetization direction is changed by first heating the magnetized domain to a temperature above its Curie temperature which removes the magnetization. The heating process is then stopped, and a new magnetized domain representative of the desired data begins to form as the temperature begins to fall. The new direction of magnetization depends on an applied external magnetic biasing field which forces the magnetization of the domain to conform to the direction of the external biasing field as the domain cools below its Curie point.
Several schemes have been used to generate the external magnetic biasing field in the past. U.S. Pat. No. 3,702,993 shows a printed wire which traces a tortuous path between discrete areas of the medium. The wire requires energization which must be controllably switched to obtain desired directions of magnetization of the domains. U.S. Pat. Nos. 3,651,504 and 3,739,394 use an external magnetic field producing coil to produce the biasing field.
Other means of providing a biasing field include the use of electric fields to obtain desired directions of electrical polarization of domains in a ferroelectric medium. U.S. Pat. No. 3,710,352 shows the application of a low voltage polarizing potential across ferroelectric media during cooling of selectively heated bit storage areas to change electrical polarization of the media. The voltage source is indicated as an external battery. U.S. Pat. No. 3,710,353 shows a ferroelectric medium in contact with a thermal capacitive region. As the capacitive region is heated with the ferroelectric medium, a voltage transfer occurs from the thermal capacitive region to the ferroelectric region resulting in a net voltage across the ferroelectric region to cause electrical polarization reversal. An energy source for the electric bias field producing capacitive region is required. A switch is also required to provide different polarizations.
A further magnetic medium structure is disclosed in U.S. Pat. No. 3,680,065 in which the Curie point of the storage layer from which data is read, is near ambient room temperature. A second layer in physical contact with the storage layer has a higher Curie temperature and has the same magnetization as the storage layer, and serves to remagnetize the storage layer by quantum exchange coupling if the temperature of the storage layer happens to reach room temperature. The second layer is not used as a biasing field to obtain desired magnetized domains, but merely to retain magnetizations obtained by common techniques.
A primary problem with the above methods of applying biasing fields is that they require a switchable energy source. At the high data recording rates required by today's storage devices, switching the sources becomes a significant problem. It takes more time to switch the source of magnetization than it does for the laser beam to pass by the domain. Thus, when writing data, at least two passes are normally required to obtain desired magnetic orientations in the magnetic domains. Also, the high switching rates produce potentially harmful electromagnetic noise.